What is Delegate?

Question

I am confused that what is the actual role of a delegate? I have been asked this question many times in my interviews, but I don't think that interviewers were satisfied with my answer.

Can anyone tell me the best definition, in one sentence, with a practical example ?

Thanks

Answer 1

I like to think of a delegate as "a pointer to a function". This goes back to C days, but the idea still holds.

The idea is that you need to be able to invoke a piece of code, but that piece of code you're going to invoke isn't known until runtime. So you use a "delegate" for that purpose. Delegates come in handy for things like event handlers, and such, where you do different things based on different events, for example.

Here's a reference for JAVA you can look at: http://www.cutthecrap.biz/software/delegates.html

In C#, for example, let's say we had a calculation we wanted to do and we wanted to use a different calculation method which we don't know until runtime. So we might have a couple calculation methods like this:

public static double CalcTotalMethod1(double amt)
{
    return amt * .014;
}

public static double CalcTotalMethod2(double amt)
{
    return amt * .056 + 42.43;
}

We could declare a delegate signature like this:

public delegate double calcTotalDelegate(double amt);

And then we could declare a method which takes the delegate as a parameter like this:

public static double CalcMyTotal(double amt, calcTotalDelegate calcTotal)
{
    return calcTotal(amt);
}

And we could call the CalcMyTotal method passing in the delegate method we wanted to use.

    double tot1 = CalcMyTotal(100.34, CalcTotalMethod1);
    double tot2 = CalcMyTotal(100.34, CalcTotalMethod2);
    Console.WriteLine(tot1);
    Console.WriteLine(tot2);

Answer 2

Wikipedia has a good article on the delegation pattern with simple and complex examples in java and php.

Answer 3

a delegate is simply a function pointer.
simply put you assign the method you wish to run your delegate. then later in code you can call that method via Invoke.

some code to demonstrate (wrote this from memory so syntax may be off)

delegate void delMyDelegate(object o);

private void MethodToExecute1(object o)
{
    // do something with object
}

private void MethodToExecute2(object o)
{
    // do something else with object
}

private void DoSomethingToList(delMyDelegate methodToRun)
{
    foreach(object o in myList)
        methodToRun.Invoke(o);
}

public void ApplyMethodsToList()
{
    DoSomethingToList(MethodToExecute1);
    DoSomethingToList(MethodToExecute2);
}

Answer 4

Think about delegate as about a simplified implementation of Command pattern.

Answer 5

Taken from here

Q What are delegates?
A When an object receives a request, the object can either handle the request itself or pass the request on to a second object to do the work. If the object decides to pass the request on, you say that the object has forwarded responsibility for handling the request to the second object.

Or, as an easy pseudo example: something sends a request to object1. object1 then forwards the request and itself to object2 -- the delegate. object2 processes the request and does some work. (note: link above gives good examples)

Answer 6

While not really a "function pointer", a delegate might look like this is a dynamic language like PHP:

$func = 'foo';
$func();

function foo() {
    print 'foo';
}

or in JavaScript you could do something like:

var func = function(){ alert('foo!'); }
func();

Answer 7

A great explanation and practical implementation of the Delegate pattern can be found in the Google Collections Forwarding Classes (also, the Decorator pattern).